1. Field of the Invention
The present application relates to a method of transmitting information in a WLAN (Wireless Local Area Network) network and corresponding WLAN communication devices and integrated circuit chips, and in particular to minimizing DC offsets therein.
2. Description of the Related Art
A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wideband radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing WLAN networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11g and 802.11a which allow data rates of 54 Mbps in the 2.4 GHz and 5 GHz spectrum, respectively. Further extensions exist.
With the growing demand for WLAN systems in the consumer market, product costs and quality have become key factors in the development of WLAN communication devices, i.e. transmitters, receivers or transceivers. Therefore, the low-IF (low-Intermediate Frequency) topology offering the prospect of integrating the RF (Radio Frequency) or IR (Infrared) front-end on-chip for reducing the production costs while providing a high operational performance has become a frequently used design for such WLAN communication devices. In a low-IF WLAN communication device operating in a reception mode, an incoming transmission signal received over a wireless communication medium, i.e. the air, is down-converted from its RF or IR carrier to an intermediate frequency of typically several hundred kHz by mixing it with an LO (Local Oscillator) signal having an accordingly selected frequency. The low-IF signal thus generated can be demodulated on this intermediate frequency or can be further down-converted to baseband after further processing, e.g., filtering.
The intermediate frequency created by the mixer is defined as the absolute value of the difference between the carrier frequency and the LO frequency. However, since the mixer does not recognize the polarity of the frequency difference between the carrier and the LO signal, down-conversion of two different received frequencies to the same intermediate frequency occurs. Apart from the wanted signal, an unwanted signal at a frequency, often referred to as the image frequency, is down-converted to the intermediate frequency.
In order to suppress the signal at the image frequency, i.e. to perform image rejection, the analytic transmission signal is converted to complex low-IF signals which are then filtered using active complex filters. In a complex filter, the filtering of positive frequencies is different from the filtering of negative frequencies. Since every frequency component of a complex signal can be written as a sum of two sequences, the first sequence having only a positive frequency component, the second only a negative, complex filters allow for eliminating the image signal in those cases where the image signal is situated on the opposite frequencies of the wanted signal.
A typical design for an RF (IR) front-end of a low-IF WLAN transceiver is shown in FIG. 1. For clarity reasons, only the signal flow in the transmission mode of the low-IF WLAN transceiver has been depicted. When the low-IF WLAN transceiver is in the reception mode, the signal flow (except for the LO signals) takes the opposite direction.
In detail, when in the reception mode, a transmission signal is provided to a complex mixer 160 for being down-converted to complex low-IF signals by being complex-mixed with the complex signals of a local oscillator 150: an LO I (In-phase) signal and an LO Q (Quadrature-phase) signal. The I- and Q-signals resulting from the complex mixer 160 are further processed in an I-path 110 and Q-path 140, respectively. This may include, e.g., amplification, filtering, or further down-conversion. Part of the I-signal resulting from the complex mixer 160 is separated before the I-path 110 is entered, complex-filtered in an active complex filter 120, and added to the Q-signal leaving the Q-path 140. Accordingly, part of the Q-signal resulting from the complex mixer 160 is split, complex-filtered in the active complex filter 130, and added to the I-signal leaving the I-path 110.
When the low-IF WLAN transceiver operates in the transmission mode, an input I-signal and an input Q-signal are provided to the I-path 110 and the Q-path 140, respectively. The signal processing in the I-path 110 and the Q-path 140 may include, e.g., amplification, filtering or up-conversion from baseband to the intermediate frequency. Part of the input I-signal (input Q-signal) is split before the I-path 110 (Q-path 140) is entered, complex-filtered in the active complex filter 130 (120), and added to the Q-signal leaving the Q-path 140 (I-signal leaving the I-path 110) to generate the combined Q-signal (combined I-signal). Subsequently, the combined I-signal and the combined Q-signal are provided to the complex mixer 160 for up-conversion to a desired transmission frequency by being complex-mixed with the LO I-signal and LO Q-signal generated by the local oscillator 150.
Complex operators like the complex filters 120, 130 and the complex mixer 160 are usually made with pairs of real operators, amplifiers, mixers and filters. The performance of the system in which these complex operators are used degrades when they are not perfectly matched. In analog integrated implementations, hence in low-IF WLAN transceivers, mismatch is unavoidable. In particular, the active complex filters 120, 130 cause the combined I-signal and combined Q-signal to suffer from a DC (Direct Current) offset when the low-IF WLAN transceiver is operating in the transmission mode. At the complex mixer 160, the DC offset causes an LO feedthrough, i.e. the transmission signal having a component at the LO frequency.
In circumstances where only frequencies within a certain frequency mask are to be used for the transmission signals of a WLAN system, the LO feedthrough can cause the transmission signal to have a component outside the allowed frequency mask. Thus, conventional low-IF WLAN transceivers often have the disadvantage of causing illegal spurious emissions.
Further, the LO feedthrough causes the transmission signal to have a higher overall signal level. This can imply that the signal level is located beyond the range of linear operation of amplifiers used for amplifying the transmission signal. This leads to degradation of the amplification efficiency. In consequence, many prior art low-IF WLAN transceivers suffer from the problem of achieving only insufficient intensity of the transmission signal at the desired transmission frequency.
In broadband systems, the carrier and LO bands of the transmission signal often happen to overlap each other since they are spaced only by the low intermediate frequency. Thus, the LO leakage gives raise to interferences between the carrier and LO bands of the transmission signal. Therefore, conventional low-IF WLAN transceivers also have the disadvantage of usually suffering from considerable deterioration of the transmission quality.